If it's still hard to see, I'll make some minor modifications to the test to make it easier to automatically recognize the correct speeds needed for specific displays. The TestUFO Blur Trail was not originally designed to reveal scanout skew (especially at default settings), but it's the best case so far (when speeding it up to near max, and when thickening line a bit, 2 pixel).
Scanout skew is an advanced topic but it is also in a scientific paper too, Page 5 of http://www.poynton.com/PDFs/Motion_portrayal.pdf
.... It's much more visible on impulsed displays (flicker displays like CRTs at low Hz (50/60) and old LED marquees) but it's still there on LCDs too (skew is so slight that it is easily hidden underneath the motion blur).
As you can see, scanout skew.
For this one in the paper, this is a bottom-to-top scan with right-to-left eye tracking. Many old LED marquees scanned upwards, and text often scrolled towards the left.
top-to-bottom scan, left-to-right eye tracking: Skews to right at top (TestUFO situation)
top-to-bottom scan, right-to-left eye tracking: Skews to left at top
bottom-to-top scan, left-to-right eye tracking: Skews to left at top
bottom-to-top scan, right-to-left eye tracking: Skews to right at top
...Now back to the relation to the thought exercise: Infinite VSYNC OFF will tilt backwards. They would cancel out the scanout skew.
Which to my brain, is fully logical, since eye-tracking position stays in sync with gametime position (which would be perfectly superfresh every scanline).
Now, even "finite-but-high" framerate (e.g. 1000fps) VSYNC OFF produces a sawtooth effect (skewing but still averages vertically perfect). This is more visible with CRT 1000fps VSYNC OFF -- on tube monitor -- during fast horizontal motion at low 50Hz or 60Hz CRT scanrates during really fast horizontal motionspeeds. While the same paper doesn't cover the "VSYNC OFF versus scanout" sawtoothing, the paper also covers "multiscanning" artifacts which creates the same sawtooth artifact too:
[RealWorld] You can see this sawtooth VSYNC+scanout "competing skew" during bright vertical edges in FPS on a CRT (60Hz + 1000fps) during really fast horziontal flicks while trying to eye-track perfectly vertical edges (dark wall edge on bright background). The higher the framerate (e.g. 2000fps Quake), the shorter the scanned persistence while bright (e.g. a bright CRT) at low Hz, the easier to see the sawtooth skew effect of the competing skew/antiskew of scanout skew versus VSYNC OFF.
[RealWorld] Even in the sawtooth, it is still cancelling each other out (As the tearline 'resets' it back to the horizontal, eye-tracking-wise), but only at intervals (locations of tearline) during the scanout. Creating the sawtooth effect. 1000fps at 60Hz on a CRT would be sawtooth vertical edges in FPS shooters (vertical average is straight, but skews between tearlines). Too much motion blur on LCDs to easily notice "VSYNC OFF versus scanout" sawtoothing artifact. And global-flash strobing (blur reduction) would disables the skewing.
[Napkin Exercise] Now, once it's infinite framerate, each shift is perfectly aligned with eye tracking position there's no sawtooth anymore as it finally perfectly balances each other out at every scan line. The currently scanned line of the output stays in sync with eye tracking.
[RealWorld] Scanout skew is so minor that we don't see it in everyday use, but it's there, with a ~2-3 degree skew during 64 pixels per frame (ppf) at 1440p 60Hz (about 3840pps) via www.testufo.com/scanskew
-> View at 60Hz from 2 meters away
. And often falls to about 1 degree skew at 120/144Hz -- I can still see it if I pay attention. Only well-constructed motion tests can make scanout skew visible on modern LCD monitors -- but once done in a properly formed motion test -- from a specific (unusual) viewing distance -- it's fairly easy to see.